Even at the present lithography capability, cooling limitations in integrated semiconductor structures can constrain the amount and type of functionality when transistors are made in an N channel technology. The cooling problem becomes more severe as the size of the integrated semiconductor structure or chip increases and the lithographic dimensions decrease. An important resolution to this problem has been found in the use of the CMOS technology wherein the devices draw virtually no standby power.
It is known that in bulk CMOS technology, an inadvertent bipolar device is formed from the several N and P type regions which acts as a parasitic silicon controlled rectifier. As long as the effective base widths of these devices are sufficiently large, the parasitic bipolar effects remain relatively unimportant in the operation of the CMOS devices. However, when forming very dense devices such as in the very large scale integration technology, such large widths waste an inordinate amount of area in the semiconductor structure or chip. To eliminate these parasitic bipolar effects, it is known to process the CMOS devices on an insulating substrate, such as a sapphire substrate, to dielectrically isolate the N channel device from the P channel device.
Processes for providing CMOS devices are taught in, e.g., U.S. Pat. Nos. 4,002,501, filed June 16, 1975, and 4,183,134, filed Dec. 11, 1978. These patents teach processes for making complementary devices which have controlled channel lengths and low gate overlap capacitance by the use of thick insulators over their source and drain region.